The invention relates generally to closed-loop voltage controlled oscillators (VCO) and more particularly to a topology to reduce phase noise in such oscillators.
FIG. 1 depicts a closed loop voltage controlled oscillator (VCO) system 10 employing positive feedback at the frequency of interest, according to the prior art. The transfer function for system 10 is given by:             Y      ⁡              (        s        )                    X      ⁡              (        s        )              =            G      ⁡              (        s        )                    1      -              G        ⁡                  (          s          )                    
where s=jxcfx89=2Πf, f is in Hz, and j=xe2x88x921. System 10 seeks to generate a sustaining, repeatable and spectrally pure tone of frequency fo when G(so)=+1. System 10 will continue to generate a steady oscillation of frequency fo providing the magnitude of the loop gain at that frequency is unity, and providing total phase shift around the loop is zero.
High-speed VCOs are commonly implemented using one or more integrated circuits (ICs) using topologies that include ring oscillators, inductor-capacitor (LC) oscillators, and composite oscillators.
A ring oscillator configuration provides a series of cascaded delay stages, where the output signal from the last delay stage is fed back to the input of the first delay stage. Total delay through the cascaded stages (plus any net inversion of the signal within the system) is designed to satisfy the criteria for sustained oscillation, as noted above. Typically each delay stage has a variable delay governed by an independent input, and oscillation frequency fo is controlled using such input to vary stage delay. The oscillation frequency for a ring counter can be tuned over a fairly wide range, perhaps 20% to 50% of the nominal center frequency. A ring oscillator topology can be used to output multiphase clock signals since the output from the nth delay stage will be offset a fixed amount of phase relative to the output from the (n+1)th delay stage. Notwithstanding that ring oscillators can exhibit a wide range of frequency and can output multiphase clock signals, ring oscillators tend to exhibit relatively high phase noise as each delay stage transfers input noise over the full bandwidth of the ring oscillator.
FIG. 2 depicts a VCO oscillator system 20 in which feedback network H(s) includes a frequency selective LC circuit. Network H(s) is included in the feedback path in an attempt to reduce VCO phase noise. The resonant frequency fo of VCO system 20 may be tuned by varying inductor L and/or capacitor C within the H(s) network. Capacitor tuning is often implemented using a varactor or voltage-controlled capacitor in H(s), and varying bias voltage coupled to the varactor. H(s) is essentially an LC bandpass filter, that advantageously can attenuate system noise at frequencies removed from the bandpass region. Thus, including an LC H(s) network in VCO 20 provides a lower phase noise VCO than a ring oscillator topology 10 such as shown in FIG. 1. Unfortunately the tuning range of a varactor-implemented LC oscillator is narrow, typically about 5% to 10% of the nominal center frequency. This limited tuning range results from the limited range of bias voltage available to tune the varactor. Further, voltage tuning an LC oscillator is inherently non-linear due to the characteristics of a varactor. Finally, an LC implemented VCO typically outputs a single phase (or its complement).
Thus far it is seen that a ring oscillator VCO can provide a wide frequency range, and multiphase output signals, but can exhibit excessive phase noise.
On the other hand, a varactor implemented LC VCO can exhibit low phase noise, but provides a relatively narrow and non-linearly tuned range of oscillation, and outputs a single phase signal.
Recent attempts in the prior art have sought to combine the best features of ring oscillator VCOs and varactor implemented LC oscillator VCOs. For example, an LC network can be provided as the load, to improve phase noise, while using a delay stage to generate multiphase output clock signals. However in such hybrid configuration, the tuning range is narrow as the LC network is used as the variable element. Attempts to more broadly tune the VCO by varying bias directly to the delay stage is problematic due to variations in output amplitude, and difficulty in sustaining oscillations. Attempts have also been made to tune the VCO by varying the coupling between stages, and by changing the weighting between stages having two different resonant frequencies. But these approaches have resulted in generation of spurious tones because the oscillator can sustain signals at a number of different frequencies. Exemplary prior art hybrid VCO circuit topologies may be found in T. P. Liu, xe2x80x9cA 6.5 GHz Monolithic CMOS Voltage-Controlled Oscillatorxe2x80x9d, ISSCC digest of technical papers, pp. 404-405, Feb. 1999, C. Lam et al., xe2x80x9cA 2.6 GHz/5.2 GHz CMOS Voltage-Controlled Oscillatorxe2x80x9d, ISSCC digest of technical papers, p. 402-403, Feb. 1999, and N. Nguyen et al., xe2x80x9cA 1.8 GHz monolithic LC Voltage-Controlled Oscillatorxe2x80x9d, IEEE J. SSC, Vol. 27, pp. 444-450, Mar. 1992.
Thus, there is a need for a VCO topology that exhibits good phase noise characteristics, while providing a good range of frequency tuning, and preferably while providing multiple phase outputs.
The present invention provides such a VCO topology.
The present invention provides a composite VCO that is based on a ring topology, and employs a bandpass frequency selective load circuit to filter phase noise. The ring topology includes a plurality of N delay stages, where each stage provides a tunable selection of at least two delays, e.g., fast delay, slow delay. Each delay stage contributes 180xc2x0/N or a 360xc2x0/N phase shift at the VCO frequency of interest. Associated with each delay stage is a delay interpolator that smoothly interpolates between the various delay paths within the stage. Each stage is coupled to a bandpass frequency selective network that acts as a load and helps filter phase noise. Feedback from the Nth delay stage to the first delay stage is inverted or not inverted, as needed, to provide the overall 360xc2x0 phase shift necessary to sustain oscillation at the frequency of interest.
Overall VCO frequency is determined by the various stage delays and by the load circuit phase response. Compared to a varactor tuned VCO, the present invention provides enhanced linearity and flexibility in controlling the frequency, which can be varied over a relatively large range. The individual stage delays preferably are varied by interpolation such that overall stability of the VCO is enhanced. Accurately controlled multiphase signals can be output from the different delay stages. The VCO may be implemented on an integrated circuit (IC).